4:00 PM - 4:15 PM
[SMP38-15] Stability and substitution mechanism of Al-bearing superhydrous phase B at the mantle transition zone and the upper most lower mantle
Keywords:superhydrous phase B, dense hydrous magnesium silicates, hydrous mineral, stability, substitution mechanism
The high pressure and high temperature experiments were conducted using a multi anvil high pressure apparatus (ORANGE-2000) installed at the Geodynamics Research Center, Ehime University, Japan. The starting materials for these experiments were powdered mixtures of MgO, Al2O3, SiO2, Mg(OH)2, and Al(OH)3 with 6 different compositions. The experiments were performed at 20 and 24 GPa at 1400 and 1600 °C. Water content of the selected samples was quantified using a SIMS installed at Hokkaido University, Japan.
Superhydrous phase B contained significant amounts of Al2O3, from15-33 wt%, and Al-bearing superhydrous phase B remained stable, even at 1600 °C and pressures of approximately 20-24 GPa. Moreover, two types of superhydrous phase Bs with different chemical compositions coexisted at 20-24 GPa and 1600 °C. The Al2O3 and H2O contents increased, and the MgO and SiO2 contents decreased as the pressure and temperature increased. The maximum H2O content of Al-bearing superhydrous phase B is 11.1(3) wt%, which is ~1.9 times larger than that of the Mg-endmember. The substitution mechanism of Al and H can be described as 2Mg2+ + Si4+ to 2Al3+ + 2H+ + Vc (cation vacancy). The crystal structures of the two coexisting superhydrous phase Bs are expected to be slightly different from each other. The present results indicate that Al-bearing superhydrous phase B can be stable in a subducted slab with a high Al content at temperatures typical of the mantle transition zone and lower mantle. Thus, water can be transported to the lower mantle by Al-bearing superhydrous phase B in a subducting slab, even at the typical mantle geotherm.